In most of the state-of-the-art communications systems, information originating from an information source is converted into bits, subsequently source and channel encoded, interleaved and then modulated for transmission over a transmission medium, which may be the space between a transmit and a receive antenna or a wired connection such as a cable or an optical fiber. Among the different modulation techniques, phase modulation has proven itself as robust and effective way of mapping information onto a carrier wave. In phase modulation, the phase of the carrier wave contains the complete information on the transmitted bits.
EDGE (Enhanced Data GSM Environment), a faster version of the Global System for Mobile Communications (GSM) wireless service, is designed to deliver data at rates up to 384 kbps and enables the delivery of multimedia and other broadband applications to mobile phone and computer users. The EDGE standard is built on the existing GSM standard. However, instead of the Gaussian Minimum Shift Keying (GMSK) modulation technique that was originally standardised for GSM, in EDGE Phase Shift Keying (PSK) with eight possible symbol constellations (8-PSK) is used. The mapping of three consecutive bits (d3i, d3i+1, d3i+2) of an encoded, interleaved bit stream bound for modulation onto one out of eight possible 8-PSK symbols 1-1 . . . 1-8 in the I/Q-plane 2 is depicted in FIG. 1, where i denotes the sequential number of the 8-PSK symbol and where the I-axis and Q-axis refer to the inphase and quadrature component of the modulated signal, respectively. All 8-PSK symbols 1-1 . . . 1-8 lie on a circle with the same radius and only differ in their phase, which is counted counter-clockwise starting from the I-axis.
When trying to demodulate a received noisy 8-PSK signal symbol-wise in order to determine the associated three transmitted bits (d3i, d3i+1, d3i+2) per 8-PSK symbol, the phase of the received signal is determined by decomposing the received signal into its inphase and quadrature components, yielding an estimated position of the 8-PSK symbol in the I/Q-plane 2 (not shown). The estimated position is compared with the possible eight positions 1-1 . . . 1-8 as depicted in FIG. 1 in order to determine which 8-PSK symbol was originally sent. However, if the symbol estimate substantially differs from the possible 8-PSK symbol positions, e.g. if the estimated symbol has a phase of 22.5°, although the transmitted 8-PSK symbol 1-1 had a phase of 0° (corresponding to the three bits (1,1,1)), it is difficult to decide whether the symbol 1—1 with phase 0° or the symbol 1-2 with phase 45° was originally sent. If it is erroneously decided that the 8-PSK symbol 1-2 with phase 45° was sent, the demodulation yields the bits (0,1,1) instead of the bits (1,1,1) that were originally sent. Thus one bit error occurs. From FIG. 1, it can be noted that neighbouring 8-PSK symbols always differ in only one bit position, in order to keep the number of bit errors as low as possible when erroneously deciding for the neighbouring 8-PSK symbol instead of the originally sent 8-PSK symbol. However, even for errors arising from the detection of erroneous neighbouring 8-PSK symbols instead of the correct 8-PSK symbol, the error probability in the bit triple (d3i, d3i+1, d3i+2) is not equal. Detecting the neighbouring 8-PSK symbol instead of the correct 8-PSK symbol may lead to a bit error in the first position of the triple (d3i, d3i+1, d3i+2) for only 4 8-PSK symbols (1-1, 1-2, 1-5 and 1-6 at 0°, 45°, 180° and 225°, respectively), may lead to a bit error in the second position of the triple (d3i, d3i+1, d3i+2) for only 4 8-PSK symbols (1-3, 1-4, 1-7, 1-8 at 90°, 135°, 270° and 315°, respectively), and may lead to a bit error in the third position of the triple (d3i, d3i+1, d3i+2) for all 8 8-PSK symbols 1-1 . . . 1-8. The third bit position in the triple (d3i, d3i+1, d3i+2) is thus much more error prone than the first and second bit position, and is thus denoted as the “weak bit” of the triple. The EDGE system (cf. technical document 3GPP TS 45.003 V5.6.0 (2000-06) from the European Telecommunications Standardisation Institute (ETSI)) allows for the multiplexing of several mobile stations onto a single uplink Packet Data Transport Channel (PDTCH). In order to control access of the different mobile stations to the PDTCH, the Uplink State Flag (USF) is used, which indicates whether or not an uplink channel is free, and, if not free, which mobile station it currently belongs to. The USF has three bits, where a “1” stands for “free”, and the remaining 7 states can be used to identify the MS that is currently using the PDTCH. The USF flag is vital for the proper functioning of the EDGE system and thus is channel encoded by a block code with code rate 1/12. In particular, the three bits of the USF are mapped onto 36 coded USF bits, and these 36 bits are distributed onto four consecutive blocks as groups of 9 bits each.
As shown in FIG. 2, which depicts the first block 3 out of four blocks of an GSM/EDGE burst, each of the blocks comprises 348 bits in total, where the coded USF bits are arranged at bit positions 168 to 173 and 176 to 178, respectively. The remaining bit positions in each block are filled with already interleaved, coded and rate-matched header and data bits. The four blocks then form a burst of length 1392 bits.
EDGE comprises thirteen different Modulation and Coding Schemes (MCS) for the PDTCH. In MCS-5 and MCS-7 (both uplink and downlink), it is proposed to avoid the transmission of coded USF bits at the third bit position in the 8-PSK triple (d3i, d3i+1, d3i+2) in order to decrease the bit error ratio of the USF. This principle is known as bit swapping. Bit swapping means that coded USF bits that correspond to bit positions in the burst that otherwise would be transmitted as the third bit in the 8-PSK triple (d3i, d3i+1, d3i+2) are swapped with bit positions that correspond to interleaved coded and rate-matched data bits and will not be transmitted as the third bit in the 8-PSK triple (d3i, d3i+1, d3i+2) . Thus USF bits are only transmitted as first or second bit in the 8-PSK (d3i, d3i+1, d3i+2), which helps to reduce the bit error ratio of the USF. As depicted in FIG. 2, the USF bits at positions 170, 173 and 176 (in the 8-PSK symbols 56, 57 and 58, respectively, shaded grey in FIG. 2) are swapped with interleaved, coded and rate-matched data bits at positions 150, 151 and 195 (not shown). It can easily be seen that, when the first bit in a burst has position 0, the USF bit positions 168, 169, 171, 172, 177, 178 (unchanged) and 150, 151 and 195 (swapped) correspond to less error prone first and second bit positions in the 8-PSK triple (d3i, d3i+1, d3i+2), because only bit positions 3k−1, with k=1 . . . 464, are mapped to the error-prone third bit position in the triple.
Swapping takes place at the transmitter. At the receiver, inverse swapping (de-swapping) of the bits yield from the demodulation of the received 8-PSK symbols is performed based on knowledge of the swapping algorithm that was used on the transmitter site. After de-interleaving, both the group of TFCI bits and the group of channel coded and rate-matched data bits are channel de-coded and then further processed.
Of course, the bit error ratio of the data bits is increased accordingly when applying bit swapping, because the error-prone third bit position in the 8-PSK triple is now assigned to the data bits more frequently. However, the performance degradation of the bit error ratio of the data bits is willingly accepted when it can be traded against the reduced bit error ratio of the by far more important USF.
A similar situation arises in the context of the Flexible Layer One (FLO), a new type of physical layer that is proposed for the GSM/EDGE Radio Access Network (GERAN, cf. technical document 3GPP TR 45.902 V6.0.0 (2003-04) from ETSI). The main advantage of the FLO is that the configuration of the physical layer (e.g. channel coding and interleaving) is specified at call setup. With FLO, the physical layer of GERAN offers one or several transport channels to the Medium Access Control (MAC) sublayer. A number of transport channels can be multiplexed and sent on the same basic physical channel, the Coded Composite Transport Channel (CCTrCH) at the same time. The configuration of a transport channel, i.e. the number of input bits, channel coding, interleaving etc. is denoted the Transport Format (TF). The configuration of the transport formats is completely controlled by the Radio Access Network (RAN) and signalled to the MS at call setup. In both the mobile station and the base transceiver station, the transport formats are used to configure the encoder and decoder units. Only a limited number of combinations of the TFs of different Traffic Channels (TrCH) are allowed. A valid combination is called a Transport Format Combination (TFC). In order to decode the received sequence the receiver needs to know the active TFC for a radio packet. This information is transmitted in the Transport Format Combination Indicator (TFCI) field. This field is a basic layer 1 header. From the decoded TFCI value the transport formats for the different transport channels are known and the actual decoding can start.
The size of the TFCI is limited to a maximum of 5 bits, allowing a maximum of 32 different TFCs on the same basic physical subchannel. In other words, for a single connection, it is proposed to have a maximum of 32 different channel coding and/or multiplexing possibilities at a time.
The TFCI is block-encoded and inserted at the beginning of a non-interleaved radio packet that further comprises the multiplexed transport channels (the CCTrCH). Each transport block of bits that is to be transmitted on a TrCH is furnished with a Cyclic Redundancy Check (CRC) attachment, channel encoded, rate-matched and then multiplexed with the other coded blocks to yield a Coded Combined Transport Channel (CCTrCH). In full rate 8-PSK channels, the non-interleaved radio packet, comprising the TFCI and the CCTrCH bits, has a total length of 1392 bits. Before 8-PSK modulation takes place, the bits of the non-interleaved radio packet are either block diagonally or block rectangularly interleaved onto I bursts, where I denotes the interleaving depth.
In the case of block rectangular interleaving, the I bursts represent a radio packet. For instance, in full rate 8-PSK channels, the K=1392 bits of the non-interleaved radio packet are then interleaved onto 4 bursts of size J=348 bits, which form the radio packet that is bound for 8-PSK modulation.
In the case of block diagonal interleaving, the bits of the non-interleaved radio packet, which comprises M=K/J non-interleaved bursts, are interleaved onto I=2*M bursts of size J bits. However, the first I/2 bursts contain only bits on the even bit positions, whereas the last I/2 bursts contain only bits on the odd bit positions. The bits of these I bursts thus have to be combined with the bits of further I bursts that stem from interleaving of the next non-interleaved radio packets onto I bursts, yielding two brimming radio packets from the two non-interleaved radio packets.
Due to the importance of the TFCI for decoding received radio packets, it is desirable to improve the bit error ratio of the TFCI. This can be achieved by bit swapping. However, in contrast to the bursts setup in the context of MSC-5 and 7 of EDGE, where the interleaving takes place before the USF bits, data and header bits are arranged in a burst and modulated, for the FLO, the bits of the TFCI and the CCTrCH are jointly interleaved.
As a consequence, in the context of MSC-5 and 7, bit swapping can be directly performed after the burst has been constructed, because it is evident which bits of the burst will be transmitted as “weak bits” of the 8-PSK modulation. For the FLO, in contrast, the joint interleaving of TFCI and CCTrCH yields I bursts, in which it is evident which bits will be transmitted as “weak bits”. However, due to the joint interleaving of TFCI and CCTrCH, the position of the interleaved bits of the TFCI within the radio packet depends on the applied interleaving scheme (block diagonal or block rectangular) and the different interleaving depths I (1, 2, 4, 8, 16) that are possible for the full, half and possible future quarter rate channels, respectively. Bit swapping thus has to cope with the different interleaving schemes and interleaving depths I.
Furthermore, it is generally preferred that bit swapping is only performed between bits that are located in the same burst. This avoids affecting the temporal diversity which is the main goal of interleaving.